The present invention relates to high resolution intravascular imaging and more particularly to ultrasound imaging and techniques for efficient and accurate image processing needed for enhanced image display.
In some types of intraluminal or intravascular ultrasound (also referred to as "IVUS") systems, an ultrasonic unidirectional exciter/detector within a catheter probe positioned within a blood vessel is used to acquire signal data from echoes of the emitted ultrasonic energy off the interior of the blood vessel. In intraluminal ultrasound imaging, the production of high resolution images of vessel wall structures requires imaging at high ultrasound frequencies. Vectors are created by directing focused ultrasonic pressure waves radially from a transducer in a catheter and collecting echoes at the same transducer from the target area. A plurality of radial vectors from the rotated transducer comprises an image frame. A signal processor performs image processing on the acquired data in order to provide a display of the intravascular image on a raster-scan display monitor. As the intravascular image displayed is used by doctors or other skilled technicians in examining the interior of the blood vessel in order to facilitate accurate diagnoses and/or to perform intricate medical procedures, it is important that the displayed image be as stable and accurate as possible and comfortably viewed. However, the movements of and within the blood vessel due to systolic and diastolic movement of the heart cause the displayed images to move on a frame-by-frame basis, thereby resulting in an unsteady view of the intravascular region. The occurrence of these movements complicates the signal processing required to provide a stable and accurate intravascular image display. Determining on a frame-by-frame basis whether and how much the image has moved is important for accurate image display or so that the image in successive frames can be accordingly adjusted to compensate for that motion.
In intravascular ultrasound imaging, three types of frequently encountered relative motions between the catheter and the blood vessel need to be analyzed: pulsation, rotation, and in-plane translation. These motions are described in the context of a plane that is in transverse cross-section to the blood vessel through which blood normally flows along the length of the blood vessel. Specifically, pulsation involves the inward and outward radial motion, uniformly or non-uniformly, of the blood vessel and blood region. Rotation involves the turning of the blood vessel and blood region with respect to the location of the center of the blood vessel. In-plane translation involves the movement within the plane of the blood vessel and blood region. Of course, some combination of these three types of motions is often encountered in intravascular ultrasound imaging and must be efficiently analyzed to provide an accurate and/or stable image display.
The problem of image displacement can be a complex matter in intravascular imaging systems, greatly affecting the quality and accuracy of the image displayed. The quality and accuracy of the displayed image is very important for doctors who may be performing intricate and often life-dependent procedures based on their real-time observations and reactions to the displayed image. For example, when the heartbeat causes blood vessels to pulsate and move in a complex manner, conventional imaging systems will show this rapid motion, making it extremely difficult for doctors to view and determine what is going on within the vessel. It is therefore very important to determine the type and quantity of the motion encountered so that an accurate image display may be provided. In addition, the image may be moved and adjusted to compensate for these motions and thereby stabilize the displayed image. It is desirable in some applications that the displayed image is sufficiently stabilized on a frame-by-frame basis with image processing analysis after data acquisition. Once the image is stabilized, then temporal filtering for blood speckle and other image enhancement techniques may be performed by existing means and techniques in order to view a corrected and filtered image.
FIG. 1 illustrates such a conventional ultrasonic imaging system that may be used for intravascular image display. As seen in FIG. 1, a specialized signal processing device 10 is used with an ultrasonic imaging system 12 including a catheter probe 13 wherein ultrasonic beams 14 are emitted by an ultrasonic transmitter or exciter 16. The ultrasonic imaging system of FIG. 1 performs data acquisition for the intravascular image in polar coordinates (r, .theta.). Radial spokes or vectors 18 of information are collected from a target 20 (the interior walls of a blood vessel) based on ultrasonic reflections at a transducer 22. Specifically, information is gathered by projecting narrow ultrasonic sampling beams 14 from exciter 16 as it is rotated (by an angle .theta.) within catheter 13 within blood vessel 20. The reflections scale in amplitude over a range and are recorded by transducer 22 as amplitude as a function of unit distance (r) along the radius of each vector. The image is representative of a cross-sectional "slice" of the structure of blood vessel 20 and includes wall structures (bloodwall interface) 26 and lumens of blood (blood region) 24, as seen in FIG. 1. This image data may be originally acquired as either analog or digital information, depending on the specific system utilized.
The data acquired is converted into pixels representing points in a scanned (swept or rotated) two-dimensional image. The pixels are assigned a value on, for example, a gray scale between black and white. Of course, in other embodiments, the assigned value may be on a color scale. After the conventional intravascular ultrasonic imaging system acquires the image data, signal processor 10 scan-converts the acquired image data and then stabilizes the rasterized scan-converted image data on a frame-by-frame basis, and then provides the raster image for viewing on a display device 30 coupled to signal processor 10. Scan conversion involves converting the digitized acquired image data into x-y rasterized image data for storing into the display memory 32 within signal processor 10. Scan conversion of data acquired in polar coordinates (r, .theta.) to pixels represented in rectangular coordinates (x, y) involves a translation (x=rcos.theta. and y=rsin.theta.) in the purely mathematical domain. Thus, each sample point acquired in polar coordinates may or may not coincide with a pixel in rectangular coordinates. Therefore, interpolation between polar coordinate sample points is often required in order to obtain each pixel in the x-y raster image. Moreover, it is apparent that some of the originally acquired data is lost upon performance of the scan conversion due to the resolution of the interpolation being insufficient to make the "polar data acquisition"-to-"rectangular scan conversion" translation transparent. Accordingly, as scan conversion quality varies, conventional intravascular ultrasonic imaging systems which provide scan-converted x-y raster image display from data acquired in polar coordinates thus may result in less accurate data representation due to the interpolation required.
Subsequent to scan-conversion (and the A/D conversion for analog acquired data), a signal processor used with conventional intravascular ultrasound imaging systems performs image processing analysis by correlation techniques on the scan-converted x-y image data in order to provide a stabilized image on a frame-by-frame basis, as discussed earlier. Examples of such correlation techniques are discussed in detail by Daniel I. Barnea and Harvey F. Silverman in an article entitled "A Class of Algorithms for Fast Digital Image Registration," on pages 179-186 of the IEEE Transactions on Computers, Vol. C-21, No. 2, February 1972, and by Petros Maragos in an article entitled "Morphological Correlation and Mean Absolute Error Criteria," on pages 1568-1571 of the IEEE Proceedings 1989 of the International Conference on Acoustic Speech and Signal Processing. Both articles are herein incorporated by reference for all purposes.
Using such correlation techniques on scan-converted x-y image data, the conventional signal processor in intravascular ultrasound systems compares successive image frames to determine what type and how much motion has occurred so that the second image frame can be re-registered with the first image frame and a corrected image can be displayed. In particular, a "region of interest" or "window" of the image frame is selected, the region of interest is compared to a "search region" larger than the region of interest, and the region of interest is moved through all possible steps in the search region to locate a specific location in the search region where there is a maximum correlation (as great as 1) with the region of interest. The position of the correlation peaks is used to determine how the image has moved. Thus, the type and quantity of the frame-by-frame motion of the image can be determined, and the second image can be re-registered to that location in the first image to thereby adjust or correct the displayed image to compensate for the motion.
In the specific imaging environment in intravascular ultrasound systems, the conventional region of interest has been rectangular (e.g., square), primarily because the various motions can be analyzed satisfactorily in rectangular (x, y) coordinates. In particular, post-scan conversion image processing analysis using the conventional rectangular region of interest in the x-y coordinate domain is able to easily handle in-plane translation by breaking down the motion into its x-component and y-component. Rotational motion using the conventional rectangular region of interest in the x-y coordinate domain is also fairly easily handled by using the arctangent function. However, pulsation is difficult and complex to analyze, as it violates the rigid body motion which conventional image processing analysis assumes. Accordingly, conventional systems with post-scan conversion image processing analysis using a rectangular region of interest must compensate for the non-rigid body motion encountered in pulsation by using computations which may result in an image display with limited accuracy. For example, the rectangular region of interest may be broken into multiple (e.g., four) rectangular sub-regions of interest within the larger region of interest and image processing performed for each rectangular sub-region of interest. The respective motions of each rectangular sub-region of interest relative to each other are used to determine how much pulsation occurred in a crude manner. Conventional intravascular ultrasound image processing systems thus may be limited to displaying a poor or inaccurate representation of the intravascular image, particularly when encountering motion such as pulsation.
From the above, it can be seen that improved methods and apparatus are needed to provide more accurate and/or stable images for display in ultrasonic intravascular imaging systems.